Phalacrostemma, MARENZELLER, 1895

Capa, María, Hutchings, Pat & Peart, Rachael, 2012, Systematic revision of Sabellariidae (Polychaeta) and their relationships with other polychaetes using morphological and DNA sequence data, Zoological Journal of the Linnean Society 164 (2), pp. 245-284 : 267-268

PHALACROSTEMMA MARENZELLER, 1895

Phalacrostemma Marenzeller, 1895: 191 ; Kirtley, 1994: 147.

Type species: Phalacrostemma cidariophilum Marenzeller, 1895 , by monotypy. Type locality: off Pelagossa Island , Adriatic Sea .

Diagnosis: Buccal flaps present, arrangement of outer paleae in a spiral on each lobe, in most species.

Description: Opercular width similar to length, operculum completely divided into two free lobes and distal disc perpendicular to operculum; eight to ten pairs of long and conical opercular papillae conical around lobes. Few simple (unbranched) tentacular filaments along margins of buccal cavity, absent in some species. Buccal flaps absent or present. Palps similar in length to operculum. Small median organ at dorsal junction of lobes. Outer paleae arranged in spirals with straight, cylindrical, smooth blades (with ornamented thecae but no denticles). Few (2–8) pairs of inner opercular paleae present, arranged in a straight line on the dorsal half of the inner margin of lobes, straight, cylindrical or slightly flattened blades and smooth margins. Two to five pairs of nuchal spines with bent tips (hooks) present and limbation in the concave margin. Neuropodia of segment 1 with one to three cirri on both sides of building organ and capillary neurochaetae. Segment 2 with two digitiform lateral lobes between noto- and neuropodia. Thoracic branchiae present. Four pairs of parathoracic segments. Parathoracic notopodia with lanceolate and capillaries alternating, neuropodia with capillaries and fine lanceolate chaetae. Abdominal dorsal branchiae absent in posterior segments.

Remarks: Monophyly of Phalacrostemma is supported by the arrangement of outer paleae in a spiral and the presence of buccal flaps present in most species. Even though this genus shares some features such as the presence of a long operculum with perpendicular disc, long opercular papillae and single tentacular filaments with Bathysabellaria , Gesaia , Mariansabellaria , and Tetreres , it has been recovered in the base of clade B after homoplasy weighting analyses supported by the length of the operculum and the presence of neurochaetae in segment 1. Phalacrostemma is a cosmopolitan group represented by ten species that live solely or in aggregations at different depths. Some of these species either lack tentacular filaments (e.g. Hartman, 1944) or have only a few present, raising questions as to how they feed. Marenzeller (1904) suggested that the type species feeds by using its simple ciliated tentacular filaments on the anterior ventral margins of the operculum together with the opercular papillae.